A hand held torque delivering power tool such as a nut runner needs to fulfil a number of criteria in order to make it efficient and agreeable to use for an operator. Firstly, it should be adapted to provide a sufficiently high torque to tighten a predetermined type of joints and it should be adapted to tighten said joints to a specific desired torque and/or clamp force.
Further, in order for the power tool to be agreeable to use for an operator, the magnitude of the reaction forces that has to be counteracted by the operator should be kept as low as possible.
The reaction forces are produced as the screw or nut is being tightened and the clamp force in the joint is produced. A nut tightening operation generally includes two phases, a first phase during which the screw is threaded into the joint and a second phase in which the screw is tightened and the clamp force in the joint is being produced. The point in time where the threading phase passes into the tightening phase is generally denoted as “snug”. It is only after snug, i.e. during the tightening phase that reaction forces will be created in the power tool. The reaction forces are created in response to the increasing torque needed to tighten the joint by rotation of the screw.
A problem that needs to be addressed in most types of hand held torque delivering power tools is to keep the counter forces as low as possible, even when a considerable torque is applied to the joint.
A solution to the above problem is presented in the patent specification U.S. Pat. No. 7,311,027 B1. In the power tool described in this specification a bit holder is driven to rotate in a first direction by means of a first motor and a flywheel is driven to rotate in the opposite direction by means of a second motor. A brake is arranged to decelerate the flywheel in response to the reaction force that are transmitted from the joint to the power tool. With an increasing reaction force, an increasing deceleration of the flywheel is achieved to compensate said increasing reaction force, such that the overall reaction force experienced by the operator will be as low as possible. A disadvantage of this arrangement is e.g. that a second motor is needed to drive the flywheel and that energy is wasted in the process.